1. Field of the Invention
The present invention relates to the field of nanomaterials such as carbon nanotubes and further to the field of phonon (heat) waveguides.
2. Related Art
Presented below is background information on certain aspects of the present invention as they may relate to technical features referred to in the detailed description, but not necessarily described in detail. The discussion below should not be construed as an admission as to the relevance of the information to the claimed invention or the prior art effect of the material described.
When a sample of material is heated on one end, energy will flow to the cold end. In solid material, heat can be carried either by mobile electrons or by atoms vibrating around their fixed, equilibrium positions. Insulating materials do not contain mobile electrons, and, as a result, only atomic vibrations can transport heat in insulators. These vibrations are not random: the atoms move collectively so that together they form waves, called phonons.
Cylindrical-structured materials such as carbon nanotubes (CNTs) or boron nitride nanotubes (BNNTs) have been known to exhibit many unique properties. These include high electronic mobility, high current-carrying capacity, high Young's modulus, high tensile strength, and high thermal conductivity1. While an enormous amount of work has been devoted to investigating their electrical and mechanical properties, few have explored the thermal behavior of an individual nanotube2-4. This is mainly due to the elaborate microfabrication processes required to construct the suspended thermal devices for an individual nanotube, as well as the difficulty in positioning a nanotube at the desired location. Recently, we have overcome these difficulties and successfully measured the isotope effect of the thermal conductivity of BNNTs. Described below is a phonon waveguide having special properties as shown by the in-situ transport measurement of an individual CNT against structural deformation. The resistance and thermopower show a reversible band-gap tuning against strain. In contrast, their thermal conductivity remains unperturbed under large deformation. More surprisingly, these features hold not only for global bending, but also for local buckling where the radius of curvature is comparable to the phonon mean free path. Furthermore, these unique thermal transport properties are also exhibited in BNNTs. Our findings suggest that nanotubes not only can be sensitive electromechanical devices but are also excellent and robust phonon waveguides with properties unknown to any other materials.
3. Specific Patents and Publications
Chang et al. “Nanotube Phonon Waveguide,” Phys. Rev. Lett., 99:045901-1 04590-4 (published on line Jul. 25, 2007) was derived from the work described here.
U.S. Pat. No. 3,626,334 to Keyes, issued Dec. 7, 1971, entitled “Electrically Variable Acoustic Delay Line,” discloses that a semiconductor can be perturbed by various methods of doping. Ion implantation, alloying, or neutron irradiation of the semiconductor can be relied upon in addition to diffusion to attain the phonon wave-guiding action, wherein the phonons are acoustic.
Pokatilov et al. “A phonon depletion effect in ultrathin heterostructures with acoustically mismatched layers,” App. Phys. Lett., 85(5): 825-827 (2004) discloses theoretically that modification of acoustic phonon spectrum in heterostructures with large acoustic impedance mismatch at the interface may lead to the strong phonon depletion in the core layer.
Schwab et al., “Measurement of the quantum of thermal conductance,” Nature, 404, 974-977 (2000) discloses that the thermal conductance of phonon waveguides in the ballistic, one-dimensional limit had been calculated using the Landauer formula. In this publication, the authors further report that they developed new fabrication techniques based on initial work on thermally isolated mesoscopic samples with integrated transducers. Their device includes a phonon ‘cavity’ (a quasi-isolated thermal reservoir) suspended by four phonon ‘waveguides’. These are fabricated from a 60-nm-thick silicon nitride membrane by electron beam lithography and pattern transfer technique.
Hone et al., “Thermal conductivity of single-walled carbon nanotubes,” Phys. Rev. B, 59, R2514-R2516 (1999) discloses that the temperature-dependent thermal conductivity κ(T) of crystalline ropes of single-walled carbon nanotubes from 350 K to 8 K. κ(T) decreases smoothly with decreasing temperature, and displays linear temperature dependence below 30 K. Comparison with electrical conductivity experiments indicates that the room-temperature thermal conductivity of a single nanotube may be comparable to that of diamond or in-plane graphite, i.e., is quite high.
Chang et al., “Solid-State Thermal Rectifier,” Science, 17 Nov. 2006:Vol. 314. no. 5802, pp. 1121-1124 discloses that high-thermal-conductivity carbon and boron nitride nanotubes mass-loaded externally and inhomogeneously with heavy molecules were produced. The resulting nanoscale system yields asymmetric axial thermal conductance with greater heat flow in the direction of decreasing mass density.
U.S. Pat. No. 4,349,796, entitled “Devices incorporating phonon filters,” discloses an acoustic superlattice of alternating layers of different acoustic impedance as a filter for high frequency phonons. Applications discussed include spectrometers, acoustic imaging apparatus, and cavity resonators.